In the art, there has been known quantum key distribution (QKD) where a shared key for encrypted data communication is securely shared by using single photons continuously transmitted between a transmission node and a reception node which are connected through an optical fiber.
In a quantum key distribution system (quantum cryptographic communication system), a generation speed of the shared key depends on a distance between sites, a quality of the optical fiber, and the like. The generation speed is in a range of several kbps to several hundred kbps. The generation speed of the shared key changes according to an external environment and disturbance of the optical fiber, installation environments of the transmission node and the reception node, and the like. Furthermore, in a case where there is an attack suspected as wiretapping, there is a possibility that the generation speed of the shared key may be sharply decreased or the generation may be stopped.
In the quantum key distribution technique in the art, it has been difficult to efficiently allocate shared keys that are continuously generated at a varying speed to a plurality of channels while synchronizing with a communication destination device.